Human skin is made up mainly of two main layers, namely the dermis and the epidermis that superficially covers the dermis. The dermis provides the epidermis with a solid support. It is also its nourishing element. It is made up mainly of fibroblasts and an extracellular matrix composed mainly of collagen, elastin and a substance known as ground substance. These components are synthesized by the fibroblasts.
The cohesion between the epidermis and the dermis is provided by the dermo-epidermal junction. This is a complex region about 100 nm thick, which comprises the basal pole of the basal keratinocytes, the epidermal membrane and the sub-basal zone of the superficial dermis. From a structural viewpoint, hemidesmosomes, into which are inserted keratin filaments (hemidesmosome-tonofilament complex), are distributed on the plasma membrane of the basal keratinocytes. Facing these hemidesmosome-tonofilament complexes are anchoring filaments that cross the epidermal basal membrane. The anchoring filaments are attached to laminin V on the epidermal side. Finally, anchoring fibrils constitute the sub-basal network. These are curvilinear structures that arise from and end on the deep face of is the basal membrane and in which are engaged fibres of collagen I, III and V.
It has been shown that these anchoring fibrils, which are entirely visible by electron microscopy, are composed of type VII collagen (referred to as collagen VII hereinbelow). The collagen VII is synthesized by keratinocytes and fibroblasts, but more substantially by keratinocytes (Aumailley M., Rousselle P. Laminins of the dermoepidermal junction. Matrix Biology, 1999, 18: 19-28; Nievers M., Schaapveld R., Sonnenberg A. Biology and function of hemidesmosomes. Matrix Biology, 1999, 18: 5-17).
Collagens are the major proteins of the extracellular matrices of the skin. To date, 20 types of collagen have been identified, and are noted from I to XX. The collagens predominantly present throughout the epidermis are collagens of the type I and III that form the extracellular matrix of the entire dermis (these collagens constitute 70-80% of the dry weight of the dermis). Moreover, collagens are not all synthesized by the same cell types: collagens of type I and III are essentially produced by the dermal fibroblasts, whereas type VII collagen is produced by two categories of cell, keratinocytes and fibroblasts. Regulation of their expression differs from one collagen to another, for example collagens I and VII are not regulated in the same way by certain cytokines; specifically, TNF-α and leukoregulin stimulate collagen VII and negatively regulate collagen I. Among the other types of collagens involved especially in ageing, mention may be made of collagens XII and VI. The collagen XII binds the fibrils of collagen Ito the other matrix compounds in the papillary dermis, and thus regulates the biomechanical properties of skin tissue: deformability and contraction of the collagen fibres by promoting sliding of the fibres relative to each other. The collagen VI facilitates, like proteoglycans, the three-dimensional arrangement of the collagen fibres. The special feature of collagen VI is its multiplicity of actions. Specifically, it binds with a large number of cells via receptors of integrin type, and with many matrix molecules (collagen IV, fibronectin, biglycan, MAGP-1). Finally, all collagen molecules are variants of a common precursor, which is the a chain of procollagen.
The dermoepidermal junction is a structure that conditions the surface state of the skin. Thus, a dermoepidermal junction with intact anchoring structures is maintained folded, thus making it possible to increase the surface area of the contact zone between the dermis and the epidermis, to promote exchanges of diffusible factors, especially between these two tissues, to reinforce their cohesion and to improve the appearance of the epidermis. In cases where the anchoring structures are impaired, in particular due to a deficiency in the synthesis of collagen VII or tenascin and/or due to ageing, this causes flattening of the dermoepidermal junction. Fewer exchanges take place, the two tissues are less solidly connected, the epidermis folds, and, as the skin is less firm and less taut, wrinkles appear and the fragility of the skin with respect to mechanical attack is increased.
Tenascin is a major constituent of the dermoepidermal junction. It is a an extracellular matrix glycoprotein, also known as tenascin-C. Its essential function lies in epithelium-mesenchyme interactions, especially during embryogenesis. In the skin, tenascin is found at the sub-epidermal level in the papillary dermis, but also around vessels and appendices. Its expression is greatly increased in situations of hyperproliferation, for instance psoriasis and tumours, but also in cicatrization. Tenascin is produced in the skin by two major cell types, the keratinocytes and the fibroblasts. One of its functions lies in cell adhesion. Specifically, the strong upregulation of tenascin in the migrating keratinocytes during cicatrization strongly suggests an essential role of adhesion of the keratinocytes to connective tissue, ensuring good dermoepidermal cohesion (Crossin K L. Tenascin: a multifunctional extracellular matrix protein with a restricted distribution in development and disease. J. Cell. Biochem. 1996, 61: 592-598; Latijnhouwers M., Bergers M., Ponec M., Dijkman H., Andriessen M., Schlkwijk J. Human epidermal to keratinocytes are a source of tenascin-C during wound healing. J. Invest. Dermatol., 1997, 108: 776-783; Steijlen P. M., Maessen E., Kresse H., Van Vlijmen I. M. J. J., Verstraeten A. A., Traupe H., Schalkwijk J. Expression of tenascin, biglycan and decorin in disorders of keratinization. Br. J. Dermatol., 1994, 130: 564-568).
With age, collagen becomes thinner and disorganised, skin cell renewal decreases, wrinkles appear on the surface of the skin, and the skin is duller and less firm. Cutaneous ageing is conditioned by genetic characteristics. Moreover, certain environmental factors such as smoking and above all exposure to sunlight accelerate it. The skin thus has a much more aged appearance on the areas exposed to sunlight, such as the back of the hands or the face. Thus, these other factors also have a negative impact on the natural collagen of the skin.
Consequently, given the important role of collagen in the integrity of the skin and in its resistance to external attacking factors of mechanical type, stimulation of the synthesis of these collagens, and in particular of procollagen I and collagens VI, VII and XII, appears to be an effective means for overcoming the signs of ageing of the skin.
To overcome the abovementioned drawbacks, to improve the appearance of the skin, to improve its mechanical properties and avoid pathologies associated with insufficiency or cell deficiency, deficiency in cell renewal or deficiency in certain compounds of the dermis or of the dermoepidermal junction, the inventors consider it to be important to develop products that are directed towards reinforcing or maintaining the role of the dermis as a support and nourishing element, the cohesion between the various layers of the skin, and more particularly the cohesion between the dermis and the epidermis, by increasing keratinocyte proliferation, stimulating fibroblasts proliferation and metabolism and stimulating collagen synthesis, in particular the synthesis of procollagen I and collagens VI, VII and XII, and increasing the synthesis of tenascin.
The epidermis, which covers the dermis and is in direct contact with the external environment, has the main role of protecting the body against the dehydration and external attack. Natural human epidermis is composed mainly of three types of cell, namely keratinocytes, which form the vast majority, melanocytes and Langerhans cells. Each of these cell types contributes, by virtue of its intrinsic functions, towards the essential role played in the body by the skin, especially the role of protecting the body against external attacking factors (the weather, ultraviolet rays, tobacco, etc.), which is known as the “barrier function”.
The epidermis is a keratinized, stratified pavement epithelium 90% made up of keratinocytes. The gradual differentiation of the cells of the basal membrane, which separates the dermis from the epidermis, towards the surface of the epidermis especially includes the differentiation of keratinocytes, which migrate towards the surface of the skin, where they desquamate.
Ageing of the epidermis is manifested mainly by a reduction in its thickness. Atrophy of the epidermis is the consequence of the slowing down of keratinocyte proliferation and of the accumulation of senescent keratinocytes. The horny layer becomes dull.
The cells constituting the epidermis are delimited by a lipid domain. In the course of differentiation, phospholipids, the role of which consists in producing the fluid structure of the cell membranes of the living layers of the epidermis, are gradually replaced by a mixture composed predominantly of fatty acids, cholesterol and ceramides (sphingolipids).
These lipids are organized in specific lamellar structures whose integrity depends not only on the quality of the fractions present, but also on their respective proportion. This lamellar structure of the lipids of the lipid domain of the epidermis is responsible for the fluidity and thus the suppleness of the skin. The lipids are also responsible for the “barrier” properties of the epidermis, particularly of the stratum corneum.
The epidermal lipids are mainly synthesized in living epidermis. They are made up mainly of phospholipids, sphingolipids, cholesterol, free fatty is acids, triglycerides, cholesterol esters and alkanes. The phospholipids are essential for the constitution of cell membranes. They play an important role in the mediation of extracellular signals and the formation of free aliphatic chains used for energy production. They constitute a reservoir of free fatty acids necessary for the constitution of the sphingolipids. The cholesterol plays a fundamental role in moisturization of the skin and in the “barrier” function of the epidermis. Free fatty acids play a major role in maintaining the lamellar structure of the lipids of the stratum corneum, and also in the constitution of cell membranes, where they are responsible for the membrane fluidity, but also for physiological processes such as the functioning of receptors or enzymatic activity.
Ceramides, which are other lipids playing a paramount role in the metabolism of the epidermis, are necessary for maintaining the multilamellar structure of the intercorneocytic lipids. They are also essential for the “barrier” function of the epidermis and for water exchanges, especially for overcoming age-related moisturization problems.
It is known practice from the literature to use agents such as ascorbic acid (vitamin C) or certain derivatives thereof to enable in particular an increase in the synthesis of ceramides (J. Invest. Dermatol. 109: 348-355, 1997; EP-1 145 706; EP-1 145 710). In vitro tests have also demonstrated, independently, that it is possible to increase the synthesis of tenascin and of collagen VII by adding vitamin C to the culture medium (EP-1 334 714).
Given the ever-increasing demand from users for improved solutions for combating the signs of biological or actinic ageing of the skin, there is a need to develop more efficient care methods.